1. Field of the Invention
The present invention relates to porous mullite-based ceramic articles and a process for the manufacturing the same. The mullite-based ceramic articles are particularly suitable for use as porous filtration devices and/or supports.
2. Discussion of the Related Art
In the field of membrane separations, thin porous membranes deposited on porous supports are widely used for microfiltration and ultrafiltration of liquid media and gas separation. The macroporous support functions to provide mechanical strength for the thin porous membrane. Porous support materials include alumina, cordierite, mullite, silica, spinel, zirconia, other refractory oxides and various oxide mixtures, carbon, sintered metals and silicon carbide.
Several considerations and limitations are important is selecting the appropriate material for the porous support. The porous support should preferably exhibit the following characteristics: (1) a total porosity, as measured by Hg intrusion of greater than 30%; (2) a high permeability; and, (3) pores exhibiting good connectivity, a greater than sub-micron average pore size and a narrow size distribution. The combined effect of these properties is that the porous support will exhibit both a good filtration efficiency and permeability such that the porous support will be suitable for most microfiltration and ultrafiltration applications. Lastly, for most applications the porous support should exhibit a sufficiently high mechanical strength (MOR) and reasonably high resistance to chemical attack. It is this last characteristic, resistance to chemical attack, that makes mullite a preferred ceramic for these filtration applications. It is known to those skilled in the art, that one conventional method for making sintered mullite structure involves firing, at about 1600.degree. C., a mixed powder of alumina (Al.sub.2 O.sub.3) and silica (SiO.sub.2), the constituent components of mullite; i.e. the reaction sintered formation of mullite bodies. Although mullite structures produced in this manner exhibit sufficient chemical resistance and mechanical strength, the mullite structures formed in this conventional manner are dense and exhibit pores of a submicron average pore size.
One reaction-sintered mullite processing innovation, enabling the formation of mullite structures exhibiting increased pore volume and pore sizes ranging from 30 to 20,000 .ANG., involved the utilization of a leaching process; U.S. Pat. Nos. 4,601,997, (Speronello) and 4,628,042 (Speronello). In the first reference, the process involves calcining kaolin clay through its exotherm without initiating the formation of substantial mullite. Thereafter, the resultant calcined clay is leached utilizing an alkaline aqueous solution so as to remove silica. Lastly, the leached kaolin clay is washed, dried and calcined at a temperature and for a time sufficient to form mullite. The second Speronello describes mixing hydrous clay, or hydrous clay and calcined clay, thermally convertible to mullite and free silica, with a fugitive binder and thereafter forming the mixture into self-supporting green bodies. The green bodies are then calcined for a time and a temperature sufficient to form mullite crystals and free silica and the calcined bodies are thereafter subject to leaching with an alkali solution to remove the free silica to create pores. As in the previous reference the mullite products produced were characterized by relatively high surface area; e.g., greater than about 15 m.sup.2 /g, high pore volume, e.g., greater than about 0.22 cc/g, and a high concentration of pores in the range of 150 to 350 .ANG. diameter.
While these Speronello references provided significant advances in the capability of the art to form porous, high strength mullite bodies, through the use of such leaching techniques, the added complexity of leaching in the processing is undesirable. Furthermore, the pore size exhibited by the mullite bodies produced by these techniques, 30 to 20,000 .ANG. is, with the majority between 100 to 600 .ANG., is less than that desirable for the aforementioned filtration applications.
Mullite formation methods involving the use of pre-reacted mullite powder represent an improvement over the aforementioned reaction sintered methods. Two such reference which disclose the use of pre-reacted mullite powder include U.S. Pat. No. 4,935,390 (Horiuchi et al.) and German Pat. No. 42 26 276 (Levkov).
The Horiuchi reference discloses a method for forming a sintered mullite-based body having improved flexural strength involving heat treating a composition of 80 to 99.1%, and 0.1 to 20%, by weight, of a mullite powder and a sintering aid, yttrium oxide, respectively. Although these bodies exhibit improved flexural strength, the use of this sintering aid results in mullite bodies which are too dense (bulk densities.apprxeq.3.0 g/cm.sup.3) to be suitable for the aforementioned filtration applications.
The Levkov reference discloses a method for the production of a ceramic sintered filter body characterized in that the starting mixture consists of 90-93% mullite, having grains of between 0.63 to 0.1 mm, an opening material, either 4-8% cork scrap or 12-16% rubber scrap, having a grain size of up to a maximum of 0.2 mm, and a binder comprising 5-7% clay and 1-3% Al.sub.2 O.sub.3 ; all in weight percent. The filter body so-formed by this method consists predominately of mullite crystals and exhibits a porosity of 50-70% by volume with pore sizes ranging from less than 30 .mu.m to up to 200 .mu.m with a high portion of the average pores ranging in size from 40-100 .mu.m. Although the porosity and pore size is much larger than that possessed by reaction sintered mullite bodies, the porosity, the pore size and pore distribution combine to result in low mechanical strength, low filtration efficiency bodies, not suitable for use as porous supports for use in microfiltration and ultrafiltration applications, specifically those applications involving pressurized liquid.
There is, accordingly, a clear need for a means for producing a porous mullite structure exhibiting an increased average pore size, a narrow pore size distribution, and high permeability, i.e., mullite bodies possessing both high filtration efficiency and high permeability suitable for use in the microfiltration and ultrafiltration of liquid media and gas separation.